Groundwater-flow characterization in a multilayered karst aquifer on the edge of a sedimentary basin in western France

On the edge of sedimentary basins, burial of geological layers can involve the formation of confined karst aquifers. In some cases, relationships between confined and unconfined karst aquifers can exist and imply an increased difficulty in delineating contributive areas of karst system outlets with accuracy. This work aims to develop a multi-disciplinary approach to highlight groundwater exchanges between multilayered karst aquifers feeding a single spring. Toulon Springs, located in western France, provides the opportunity to study relationships between two multilayered karst aquifers. Hydrograph and chemograph analysis coupled with principal component analysis (PCA) are used to evaluate groundwater-flow origins on the northern edge of a sedimentary basin. Natural hydrodynamic responses (baseflow recession analysis, auto-cross correlation function) show an important volume of water located in a saturated zone of the Toulon karst system. Significant contrasts in water quality as a function of the hydrological regime (flood, recession, and baseflow regime) and comparison with shallow and deep water show the participation of a deep aquifer supporting the flow of Toulon Springs.